It is known to separate one fluid from a mixture of fluids by using a membrane which is more permeable to the one fluid than the others of the mixture. The fluid mixtures are brought into contact with one side of the membrane with a pressure differential being maintained across the membrane, and a permeable fluid will permeate the membrane to the other side thereof, and thereby become separated from the fluid mixture.
In the present instance, moisture or other rapidly diffusing gases are separated from other less rapidly diffusing gases. For instance in the dehydration of air or natured gas, the rate of permeation of the moisture through a membrane is a function of the difference in partial pressures of the moisture on opposite sides of the membrane. If these partial pressures are the same, none of the moisture will permeate the membrane. This can present a problem when it is desired to dehydrate a gaseous mixture containing relatively small amounts of moisture.
Because of the high permeation selectivity of water, the water vapor partied pressure on the permeate side of the membrane can approach that of the feed side, so that the permeation rate of the water becomes very low. One method of solving this problem is to use a sweep gas on the permeate side. The sweep dilutes the permeated water concentration and lowers the water vapor partial pressure on the permeate side, thereby producing higher permeation rates of water.
Several attempts have been made to provide sweep gas to the permeate side of the membrane so as to allow more of the water to permeate the membrane. For instance in U.S. Pat. No. 3,536,611, a membrane device is disclosed which uses a sweep stream to sweep a permeated fluid from a bundle of hollow membranes through which the fluid is permeated. The sweep streams are brought in from opposite ends of the bundle of fibers, and it is obvious that exterior piping and valves, etc. are required for directing the sweep streams into the device. Such piping and valves add to the cost and complexity of the system.
In U.S. Pat. No. 4,687,578, a major fraction of the fiber length is coated to reduce the permeation rate of the gas carrying the moisture, and the remaining fiber length at the non-permeate end of the module is not coated or just partially coated, wherein the gas carrying the moisture permeates rapidly, and thereby provides a sweep gas on the permeate side countercurrent to the feed gas. However, it is difficult to coat fibers with selected coatings, such as defect repair material as defined in U.S. Pat. No. 4,230,463, on only a part of the fiber leaving the rest of the fiber uncoated.
In U.S. Pat. No. 4,783,201, the membrane was not coated, and in an attempt to provide a sweep on the permeate side, controlled porosity of the membranes was effected along the length of the fiber. Here again, a special procedure was required to treat the hollow fiber membrane material to alter the membrane porosity. Low porosity is desired at the moist feed end of the module and higher porosity at the product end to allow high permeation rate of the gas carrying the moisture, thereby providing a countercurrent sweep on the permeate side.
Furthermore, if the fiber used in the module is produced by a process that results in low permeability for the carrier gas, then providing a sweep via permeation through the fiber, as described in the above patents, U.S. Pat. No. 4,687,578 and U.S. Pat. No. 4,783,201, may not be a feasible option. Examples of fiber production processes that can result in low permeability for the carrier gas include wet spinning phase inversion, solution coating, interfacial polymerization, among others. The present invention provides a module with an internal permeate-side sweep, when the fiber to be utilized has a low permeability for the carrier gas.
In the ordinary production of hollow fiber membranes for gas separation modules, the fiber is generally post-treated with a coating material to repair the defects in the membrane and thereby increase its selectivity for gas separation. Heretofore, membranes which have been treated for defect repair have been found to be unsuitable for use in dehydration processes, because no method of satisfactorily providing the sweep gas had been developed. The previous methods are inadequate for the reasons described above.
The present invention provides a hollow fiber membrane module, which provides an adequate internal sweep on the permeate side of the fibers in an economical and easily-produced method.